Seismometer



Dec. 1,-1942. H. w. WASHBURN SEISMOMETER Filed July 28, 1939 F'J'El INVENTOR. 15%?040 W MSHBUAN BY I o ATTORNEYS.

- Batented Dec. 1, 1942 STA to Consolidated Enc m e Pasadena, Calif., a corporation oil @alirou. .51

Corporation,

Application July 28, 1939, Serial No. 28?,076

9 Claims. (Cl. 177 352) I My invention relates to an instrument for detecting vibrations, particularly seismic waves.

One object .of my invention is to provide a seismometer with a low resonant frequency.

Another object of my invention is to provide a means for controlling the relative effects that the magnetic and spring forces have in determim ing the frequency of a seismometer.

' Another object of my-invention is to provide a reluctance type seismometer in which the resonant frequency is affected only slightly by'the magnetic circuit.

Another object is to provide a variable reluctance type seismometer.

Another object is to provide a seismometer having high sensitivity per unit weight.

Still another object is to provide aseismometer having substantially uniform response throughout the frequency range of interest in seismic surveying. I

Other objects of my invention will be apparent or will be specifically pointed out in the description forming apart of this specification.

Referring to the drawing:

Fig. lie 9. plan view of the working parts of one preferred form of my invention taken on the line |-l of Fig. 2.

Fig. 2 is a vertical section of one form of my seismometer taken on the line 2-2 of Fig. 1.

Fig. 3 is a vertical section of another form of my invention also taken on the line 2-2 of-Fig. 1.

Referring to Figs. 1 and '2 parallel permanent magnets I, 2 have soft iron cross pieces 3, a, butted against the ends thereof. Magnets I, 2 and cross pieces 3, i, are secured to brass end plates 5-5. Yoke type magnetic poles 6 and I are rigidly mounted on cross pieces 3, 4, respectively, facing inwardly and forming central air gaps. Each of said poles has a'pair of pole tips symmetrically located but differently spaced with respect to a reference plane perpendicular to the plane of section drawn in Fig. 2 and passing through a center line X-X drawn through the poles. As shown, pole tips 6a and 6b of pole B are spaced further apart than pole tips la and lb of pole 1. Thus, pole tips In. and lb opposite pole tips 611 and 6b respectively are symmetrically offset therefrom.

Flanges, 8a and 8b of the .bobbin type armature 8 are respectively located in the cylindrical shaped air gaps formed by pole tip pairs 6a, 1a, and 6b, lb. Said armature is centrally supported by guide springs 9 and I secured to a frame formed by magnets I and 2' and crosspieces 3 and t and end plates -5.

' interest.

' weight and a system in whic The fields of magnets i and point in the r direction so that a high concentration or Kill passes through the pole tips 3h, la, it and. the armature t. Said pole tips (it, "is, and. it; an said armature flanges Ba and dime'csio o and positionedso that in the can ihrium position flux flows from pole tip to, to so a tip la. through. flange 8a and a substantially equal amount of flux flows from pole tip 6b to pole tip lb through flange 86*. Thus in the equilibrium position substantially no flux flo through. the central core to of armature 8.

A coil l t surrounding core and secured to the frame is adapted to detect changes in flux through said core. I L L Whenever the frame moves "a." respect to the armature the flux through changes an a corresponding electromotivc force is induced in coil it. Thus vibrations reaching the frame produce corresponding electrical rates in coil M. My invention may be used a seisrnometer to detect earth vibrations produced either s on taneously or artificially. In. seismic prospecting earth waves lying in the range of frequencies from about 25 to 150 cycles are of particular My seisinometer may he used to reproduce these waves faithfully at all the frequencies of. interest. In order to achieve faithfulness of reproduction, I utilize a pickup coil 1!! secured'to the frame, a low resonant frequency for the mov ing armature 8, and a low inductance winding, By using a pickup coil so mounted I am able to use electromagnetic damping by short circuit-tug said 0011 with an appropriate damping resistor and I also attain a large ratio of the weight of the case to the weight of armature.

Low inductance per unit turn is obtained by utilizing relatively thin pole tips to, iib, la, and "I I1. Such low inductance aids in producing uniform response at high frequencies.

To achieve a low resonant frequency, prefer: ably lower than the frequency of any wave .i. desire to record I use an a iture having low 1e force constant of the moving armature is 19*.

The force constant affecting the resonantire quency of my seismometer-is produced by springs 9 and it and magnetic forces between the pole tips and the armature. As is well known the resonant frequency of a vibrating system decreases as the force constant decreases.

To bring about 'a decrease in force constant I use weak springs at it and and I introduce auxiliary pole segments i?! and. which coast with armature 8 to oppose magnetic forces produced between armature B and pole tips 6a., (5b, Ia and lb when said armature is displaced from its equilibrium position.

As will be clear to those skilled in the art, when armature B is displaced upward or downward from its equilibrium position, the magnetic Forces between the pole tips 6a., 6b,- !a, and lb and said armature tend to restore said armature to .ts equilibrium position. Pole segments 92 and I3, however, attract armature 8 and consequently zend to draw the armature further from its equilibrium position. By properly proportioning :hese two counteracting forces the magnetic stiffness is desired. It the forces producedby segments I2 and I3, almost but not completely, counteract zhe forces produced by pole tips 6a, lib, la, and lb, ;he resonant frequency of the system depends alnost entirely on the stiffness of springs ii and ill.

The combination of variable area and variable zap characteristics and the attachment of the of the system may be made as small 1 formed between. pole pieces, characterized by "an )ickup coil to the frame instead of the moving iart makes possible:

1. A low inductance winding compared with he necessary damping resistance;

2. Electromagnetic damping;

3. Light total mass;

4. Large ratio of moving mass to stationary nass; thus making possible fiat steady state reponse over a wide frequency band and also a imple transient response.

To prevent stray pickup from afiecting the ystem seriously, I use a system comprising four 301% tips on each side and two armatures d 8 secured together as in Fig. 3 and two pickup :oils ii and 28 connected in series. -I utilize wide pacing between pole tip pairs ta, lib, and lo, "id

,nd narrow spacing betweenpole tip pairs tic, d and la, l b, so that when the coils are contested to add efiects of case vibration, any elecromotive forces induced from external sources 7111 be cancelled.

Basically my seismometer mechanism is a varible area type using opposed pairs of like pole ms symmetrically arranged about a central lane. In the preferred form of my inventim 1 which the magnetic stiffness is variable it com me eifects of both variable area and variable gap vpe instruments. Obviously in outdoor use, an rrclosing casing of convenient form may b tilized.

From the foregoing it is seen that I have proided a seismometerwhich is simple and rugged i construction, sensitive and adapted to reprouce seismic waves faithfully. lit is obvious that many modifications and varinons of my invention, as hereinabove set forth, aybe made without departing from the spirit my invention. It is therefore to be understood lat my invention is not to be limited to the speflc details shown and described but only bythe upended claims. I claim: 4 l v 1. In an electro-mechanical transducer having magnetic system and a magnetic armature corerating with said magnetic system to produce st magnetic forces acting on the armature and nding to restore the armature to a neutral po- ;ion when displaced therefrom in a predeterlned direction, a pickup coil surrounding said mature and responsive to flux changes through e armature, resilient means supporting said mature on said magnetic system also tending restore said armature to said position after one of the displacement therefrom in said direction, and additional magnetic means cooperating with said armature and said magnetic system and positioned 'to provide a second magnetic force to a least partially counteract said restoring forces.

2. ,In an elcctro-mechanical transducer having a magnetic system symmetrical about a reference plane and a magnetic armature symmetrical about a refcrence plane parallel to the'first plane, said armature being displaceable along an axis perpendicular to the planes and positioned in magnetically self-centering relation to the magnetic system, resilient means supporting said armature in said magneticsystem for restoring said armature to said position after displacement therefrom, and additional magnetic means positioned to at least partially counteract the restoring forces of said resilient means.

3. An electronnechanical transducer of a type having an armature movable in a magnetic gap armature pole piece relationship effectively producing during armature movement a variable field intersection area in a. surface parallel to the direction of armature movement resulting in a force tendingto center said armature with respect to pole pieces, and a gap of variable thickness along a line parallel to the direction of armature movement resulting in an alined force tending to counteract said. centering force.

4. In an eiect1ro-*nagnetic one trans suspendoiz opposite ction parallel ble field in- I .1 pin the pole tips, a T magnetic pole tips secured to liehts and forming with the other element a structure having two gaps of variable thick; as in a direction. parallel to the displacement direction and arranged so that one airgap thickness increases the other decreases with displacement.

5. An electrounechanical transducer compris-' ing' a magnetic system having a pair of spaced north pole tips and a pair of differently spaced south pole tips, both pairs being svmmetrlcalli disposed with respect to a common reference plane, an armature of soft magnetic material resiliently supported in the gaps between the D0 tips. and displaceable along an axis normal to said reference plane, said armature comprising a central core having an axis coinciding with the,

Pair of an:

displacement axis, a pair of flanges on the ends of the core in planes normal to said axis and spaced apart adistance intermediate the spacing between the pairs of pole tips, each flange overlapping inner portions oi. the outer pole tips of the same polarity and outer portions of the other pairs of pole tips, the amounts of overlapping varying "with the displacement; and a, pickup coil surrounding said core.

.6. An electrmmechanical transducer comprising a magnetic system having a pair of spaced north pole tips and apair of differently spaced south pole tips, both pairs being symmetrically disposed with respect to a common reference plane, an armature of soft magnetic material recentral core having an axis coinciding with the displacement axis. a pair of flanges on the ends of the core in planes normal to said axis and spaced apart a distance intermediate the spacing between the pairs of pole tips, each flange bein positioned with respect to the pole tips to provide a magnetic flux path from a north pole tip to a south pole tip on the same side of said reference plane and also a magnetic flux path from said north pole tip to the other south pole ti through said core, the magnetic forces between said armature and said magnetic system tending to restore said armature to a neutral position in which the armature is symmetrically located with respect to said reference plane when the armature is displaced from said neutral position, auxiliary flux distribution means positionedin such relation to the armature and said magnetic system as to produce magnetic forces on the armature at least partly counteracting the aforesaid magnetic forces, and a pickup coil surrounding said core.

7. An electro-mechanical transducer comprising a magnetic system having a pair of spaced north pole tips and a pair of differently spaced south pole tips, each of the pairs being symmetrically disposed with respect to a common reference plane, an armature of soft magnetic material resiliently supported in the gaps between the pole tips and displaceable along an axis normal to said reference plane, said armature comprising a central core having an axis coinciding with the displacement axis, a pair of flanges on the ends of the core in planes normal to said axis and spaced apart adistance intermediate the spacing between the pairs of pole tips, each flange being positioned with respect to the pole tips to provide a magnetic flux path from a north pole tip to a south pole tip on the same side of said reference plane through the flange and another flux path through said armature core from a north pole tip on either side to the south pole tip on the other side of said reference plane, and a pickup coil surrounding said 9. In an electromechanical transducer including a magnetic system having poles on opposite sides of a reference plane, an armature alsohaving flux bearing sections on opposite sides of said reference plane, resilient means supportin said armature on said magnetic system and constraining the armature movement to a direction perpendicular to said reference plane, said poles and said armature cooperating to produce a magnetic force tending to restore the armature to a neutral position when displaced therefrom, and auxiliary flux distribution means positioned on opposite sides of said plane and cooperating with said armature to produce a magnetic force on the armature in an opposite direction to said aforesaid magnetic force during armature displacement.

HAROLD w. WASHBURN. 

